mx.SMS-USLUGI.RU has no CNAME records.
Question and response
Question
dig mx.SMS-USLUGI.RU. NS
Answer
The response has no answer records.
Authority
sms-uslugi.ru. 60 SOA ns1.zcdn.ru. adm.sms-uslugi.ru. 2025121806 900 900 1800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0